(1) Field of the Invention
The present invention relates to a method for developing electrostatic images. More particularly, the present invention relates to a method for forming a toner image at a high density without fogging by developing an electrostatic image by a magnetic brush.
(2) Description of the Prior Art
In the electrophotographic process using a two-component type magnetic developer, an electroscopic toner is mixed with a magnetic carrier, the resulting two-component type composition is supplied to a developing sleeve having a magnet arranged in the interior thereof to form a magnetic brush formed of this composition, and this magnetic brush is brought into sliding contact with an electrophotographic photosensitive plate having an electrostatic latent image formed thereon. The electroscopic toner is charged with a polarity reverse to the polarity of the electrostatic latent image on the photosensitive plate by friction with the magnetic carrier, and particles of the electroscopic toner on the magnetic brush are stuck to the electrostatic latent image by Coulomb force to effect development of the electrostatic latent image. On the other hand, the magnetic carrier is attracted by the magnet arranged in the interior of the sleeve, and the polarity of the magnetic carrier is the same as the polarity of the charge of the electrostatic latent image. Accordingly, the magnetic carrier is left on the sleeve.
The charged toner particles are electrostatically attracted to the electrostatic latent image and also are electrostatically attracted to the magnetic carrier, and in the case where toner particles are excessively attracted to the electrostatic latent image-bearing photosensitive plate, fogging is caused, but if toner particles are excessively attracted to the magnetic carrier, such troubles as reduction of the image density and reduction of the developing efficiency are caused. This threshold value for the development is controlled by adjusting the bias voltage between the photosensitive plate and the sleeve, but adjustment of this bias voltage is limited as a matter of course. For example, if a high bias voltage is applied to produce fogging-preventing development conditions, the density of the formed toner image is generally low.
Also in case of two-component type developers, it is empirically known that at a high toner concentration fogging is readily caused and at a low toner concentration the image density is reduced. Accordingly, the toner is ordinarily mixed with the magnetic carrier so that the toner concentration is 5 to 10% by weight, and the resulting mixture is used for the development.